The involvement of selenium (Se) in biological systems must be considered in two contexts, its nutritional essentiality as a trace element and its toxicity at higher levels. The relationship of these biological aspects of Se to the many known metabolic analogies between sulfur-(S-) and Se-containing compounds is not clear. Based on our recent discovery that Se is incorporated into tRNA bases by E. coli the following questions will be pursued: (1) Is Se a common trace constituent of tRNA regardless of the source? This will be investigated in two ways. First, tRNA samples from various sources (plant, animal and microbial) will be subjected to neutron activation analysis for Se content. Second, radioactive Se compounds will be administered to various organisms followed by assay of extracted tRNA for radioactivity. (2) What is the chemical nature of the selenobases? This will be studied by chromatographic and electrophoretic comparison of radiolabeled selenobases obtained as above with standard selenobases obtained by chemical synthesis. (3) Does Se follow S pathways into tRNA or does a unique pathway exist? To study this, a cell-free system for tRNA thiolation will be isolated from E. coli and tested for Se transfer from selenocysteine as well as for competition between selenocysteine and the normal substrate, cysteine. (4) What are the functional consequences of high Se content in tRNA? Using tRNA extracted from E. coli grown in high Se medium, effects on aminoacylation and ribosome binding will be studied.